The present invention is directed to a mode matching gain element connecting two single mode optic fibers, and to a method for performing such mode matching. More specifically, the invention is directed to a phase conjugating structure for mode matching in a super luminescent diode cavity used as a gain element between two single mode optic fibers.
Optical circuits often employ devices such as super luminescent diode cavities as gain elements to boost an optical signal traveling through the circuit. These gain elements typically have an input and an output to which optic fibers are connected via a coupler.
The purpose of the gain element is to take the signal entering from the input fiber, boost it, and output the signal to the output fiber. To do this, the gain element must contain a sufficient volume of gain material so that the desired amount of amplification of the signal is obtained. Since the optic fibers have a very small diameter, the gain element usually has a tapered shape, starting at the coupler with a diameter equal to that of the optic fiber, increasing to a much larger diameter, and then decreasing again to the diameter of the output fiber.
A problem occurring with this conventional configuration is shown schematically in FIG. 1. Gain element 14 has an input optic fiber 10 and an output fiber 12 connected to it, so that an incoming wave front 16 propagates through gain material 18 from input optic fiber 10. The shape of the incoming wave front 16 however, does not match a shape of an outgoing wave front 17 that the signal must assume in order to exit the gain element 14 through output optic fiber 12. This mismatch causes losses to occur in the gain element.
The severity of this problem can be reduced by making gain element 14 into a very long tapered shape, where the diameter of the taper increases slowly from the diameter of input fiber 10, to a maximum diameter, and back to the diameter of output fiber 12. In this manner, incoming wave front 16 can change adiabatically, or with little loss of energy, from the shape of wave front 16, to essentially a straight line wave front at the largest diameter, and then to the shape of outgoing wave front 17 required to exit through output fiber 12. The problem with this design is that the taper for the gain element is excessively long, and takes up a large amount of space in the optical circuit.
In addition, conventional gain elements, also known as gain tapers, may support multiple nodes of the laser light being amplified therein. In particular, when the input and output optic fiber are mono-mode fibers, the additional modes supported by the gain element result in leakage of radiation from the gain element and additional losses of signal.
In view of the foregoing, there is a need for a single mode gain element for optical circuits that has a small size and that matches the mode and the wave front of the incoming signal to that of the outgoing signal, thus avoiding the excessive losses associated with conventional devices.
The present invention is directed to a mode matching gain element for optical networks that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention.
To achieve these and other advantages in accordance with the present invention, the invention is a single mode gain element of an optical system having an input and an output for an optical signal, comprising a gain medium having a geometry adapted to amplify the optical signal and to support a desired optical mode of the optical signal, and a phase conjugating structure, disposed in a path of the optical signal to time-reverse the optical signal between the input and the output.
In another aspect, the invention is a super luminescent diode cavity, comprising input and output optic fibers capable of supporting a signal of a single optical mode, a gain region to amplify the signal, said gain region supporting the single optical mode, and a phase conjugating region to time reverse the signal between the input and the output optic fibers.
In yet another aspect, the invention is a method of amplifying a signal between an input mono-mode optic fiber and an output mono mode optic fiber, comprising passing the signal through a single gain region, said gain region being selected to support an optic mode substantially identical to the mode of the input and output optic fibers, and performing a time-reversal of the signal, to match a wavefront of the signal at the input fiber to a wavefront of the signal at the output fiber.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.